The invention relates to twisted nematic (acronym TN) and supertwisted nematic (acronym STN) liquid crystal displays having very short switching times and good steepness and angular dependence and to the novel nematic liquid crystal mixtures employed therein.
TN displays are known, e.g. from M. Schadt and W. Helfrich, Appl. Phys. Lett., 18, 127 (1971). STN displays are known, e.g. from EP 0 131 216 B1; DE 34 23 993 A1; EP 0 098 070 A2; M. Schadt and F. Leenhouts, 17th Freiburg Conference on Liquid Crystals (8-10/04/87); K. Kawasaki et al., SID 87 Digest 391 (20.6); M. Schadt and F. Leenhouts, SID 87 Digest 372 (20.1); K. Katoh et al., Japanese Journal of Applied Physics, Vol. 26, No. 11, L 1784-L 1786 (1987); F. Leenhouts et al., Appl. Phys. Lett. 50 (21), 1468 (1987); H. A. van Sprang and H. G. Koopman, J. Appl. Phys. 62 (5), 1734 (1987); T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (10), 1021 (1984), M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (5), 236 (1987), and E. P. Raynes, Mol. Cryst. Liq. Cryst. Letters Vol. 4 (1), pp. 1-8 (1986). The term STN herein comprises any relatively strongly twisted display element having a twist angle amounting to between 160xc2x0 and 360xc2x0, for example the display elements according to Waters et al. (C. M. Waters et al., Proc. Soc. Inf. Disp. (New York) (1985) (3rd Intern. Display Conference, Kobe, Japan), the STN LCDs (DE-A-35 03 259), SBE LCDs (T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021), OMI LCDs (M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (1987), 236, DST LCDs (EP-A 0 246 842) or BW STN LCDs (K. Kawasaki et al., SID 87 Digest 391 (20.6)).
STN displays in particular are distinguished, compared with standard TN displays, by considerably better slopes of the electro-optical characteristic curve and the attendant better contrast values and by a significantly reduced angular dependence of the contrast. Of particular interest are TN and STN displays having a very short switching time, especially at lower temperatures. To achieve short switching times it has hitherto been the practice to optimize the rotational viscosities of the liquid crystal mixtures, by employing usually monotropic additives having a relatively high vapour pressure. The switching times achieved were not, however, adequate for all applications.
To achieve a steep electro-optical characteristic curve in TN and STN displays, the liquid crystal mixtures should have relatively large values of K3/K1 and relatively small values of xcex94∈/∈1.
Beyond optimized contrast and optimized switching times, such mixtures are subject to further important requirements:
1. wide d/p window
2. high chemical long-term stability
3. high electrical resistance
4. low frequency dependence and temperature dependence of the threshold voltage.
The parameter combinations achieved are still inadequate by a long shot, especially for high-multiplex, but also for low- and medium-multiplex STNs (1/400). One reason for this is that the effects of material parameters on the various requirements pull in opposite directions.
There is therefore still a great need for TN and STN displays, especially for high-resolution displays (XGA), having very short switching times in conjunction with a wide operating temperature range, a steep characteristic curve, good angular dependence of contrast and low threshold voltage, which comply with the above-specified requirements.
It is an object of the invention to provide TN and STN displays which do not have the above-specified draw-backs or which have them only to a lesser extent and at the same time have low threshold voltages and very good steepness.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
We have now found that this object can be achieved if nematic liquid crystal mixtures are employed which comprise compounds of formula IA 
in conjunction with alkynyl-bonded compounds of formula IB, 
where
R1 and R2 each, independently of one another, are an alkyl, alkoxy or alkenyl group having from 1 to 12 C atoms, with the additional option of 1 or 2 non-adjacent CH2 groups being replaced by xe2x80x94Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94OCOxe2x80x94 or xe2x80x94COOxe2x80x94 in such a way that O atoms are not linked directly to one another,
R3 is an alkenyl group having from 2 to 7 C atoms, and
L is H or F.
The use of the compounds of formulae IA and IB in the mixtures for TN and STN displays according to the invention results in
low viscosities,
wide nematic phase ranges,
a steeply sloping electro-optical characteristic curve, and
low threshold voltages.
The compounds of formulae IA and IB, in particular, cause a distinct increase in the steepness of TN and STN mixtures whilst at the same time reducing the threshold voltage.
The mixtures according to the invention are further distinguished by the following advantages:
they exhibit low temperature dependence of the threshold voltage and the operating voltage,
they have short switching times even at low temperatures,
they result in long persistence times in the display at low temperatures.
The invention therefore relates to a liquid crystal display comprising
two substrates which, together with an edging, form a cell,
contained in the cell, a nematic liquid crystal mixture having positive dielectric anisotropy,
electrode layers having alignment layers on the insides of the substrates,
a pre-tilt angle between the longitudinal axis of the molecules at the surface of the substrates and the substrates of from 0 degrees to 30 degrees, and
a twist angle of the liquid crystal mixture in the cell which, from alignment layer to alignment layer, amounts to between 22.5xc2x0 and 600xc2x0,
a nematic liquid crystal mixture comprising
a) 20-90 wt % of a liquid-crystalline component A, consisting of one or more compounds having a dielectric anisotropy of more than +1.5;
b) 10-80 wt % of a liquid-crystalline component B, consisting of one or more compounds having a dielectric anisotropy of between xe2x88x921.5 and +1.5;
c) 0-20 wt % of a liquid-crystalline component C, consisting of one or more compounds having a dielectric anisotropy of less than xe2x88x921.5 and
d) optionally an optically active component D in such an amount that the ratio between layer thickness (spacing of the substrates) and natural pitch of the chiral nematic liquid crystal mixture is from about 0.2 to 1.3,
characterized in that component A of the mixture comprises at least one compound of formula IA, 
xe2x80x83where
R3 is an alkenyl group having from 2 to 7 C atoms,
xe2x80x83and the mixture comprises at least one compound of formula IB, 
xe2x80x83where
R1 and R2 each, independently of one another, are an alkyl, alkoxy or alkenyl group having from 1 to 12 C atoms, with the additional option of 1 or 2 non-adjacent CH2 groups being replaced by xe2x80x94Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94OCOxe2x80x94 or xe2x80x94COOxe2x80x94 in such a way that O atoms are not linked directly to one another, and
L is H or F.
The invention also relates to corresponding liquid crystal mixtures for use in TN and STN displays, especially in medium- and low-multiplexed STN displays.
Particularly preferred compounds of formula IA are those of the following formulae 
where R3a is H, CH3, C2H5 or n-C3H7.
Particularly preferred compounds of formula IB are those where L is F.
The component A, in addition to the compounds of formula IA, preferably further comprises one or more compounds of formulae II and/or III, 
where
R is an alkyl, alkoxy or alkenyl group having from 1 to 12 C atoms, with the additional option of 1 or 2 non-adjacent CH2 groups being replaced by xe2x80x94Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94OCOxe2x80x94 or xe2x80x94COOxe2x80x94 in such a way that O atoms are not linked directly to one another, 
xe2x80x83and 
xe2x80x83each, independently of one another, are 
xe2x80x83or 
L1 to L6 each, independently of one another, are H or F,
Z1 is xe2x80x94COOxe2x80x94, xe2x80x94CH2CH2xe2x80x94 or a single bond,
Z2 is xe2x80x94CH2CH2xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94 or a single bond,
Q is xe2x80x94CF2xe2x80x94, xe2x80x94CHFxe2x80x94, xe2x80x94OCF2xe2x80x94, xe2x80x94OCHFxe2x80x94 or a single bond,
Y is F or Cl,
a is 1 or 2, and
b is 0 or 1,
where, regarding formula II, the compounds of formula IA as specified above are excluded.
Preferred compounds of formula II correspond to the subformulae IIa to IIh: 
where R, L1, L2 and L3 have the above-specified meanings and, in the case of compounds of formula IIb where L1 and L2 are H, R is an alkyl or alkoxy group.
Particularly preferred are mixtures comprising one or more compounds of the following subformulae 
where R has the above-specified meaning.
Also preferred are mixtures comprising one or more compounds of formula IIh, where L2 is H and L1 is H or F, especially F.
In a particularly preferred embodiment, component A additionally comprises compounds of formulae AI to AIV: 
where
R is an alkyl, alkoxy or alkenyl group having from 1 to 12 C atoms, with the additional option of 1 or 2 non-adjacent CH2 groups being replaced by xe2x80x94Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94OCOxe2x80x94 or xe2x80x94COOxe2x80x94 in such a way that O atoms are not linked directly to one another, 
xe2x80x83and Q1 each, independently of one another, are 
Z3 is 
xe2x80x83xe2x80x94CH2CH2xe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94 or a single bond.
Preferably, the mixtures according to the invention comprise one or more polar compounds having a high clearing point, selected from the group consisting of the compounds AIV1 to AIV4: 
In the compounds AIV1 to AIV4, the 1,4-phenylene rings can also be substituted laterally by one or two fluorine atoms. Preferred compounds of this type are the compounds of formulae AIV1-1, AIV1-2 and AIV1-3: 
In the mixtures according to the invention which comprise compounds of formulae AIV1 to AIV4, the proportion of these compounds is preferably from about 2 to 25%.
Preferred compounds of formula III correspond to the subformulae IIIa-IIIv: 
where R has the above-specified meaning and L3 and L4, independently of one another, are H or F.
Particularly preferred among the compounds of formulae IIIa to IIIv are those where L4 is F, also those where L3 and L4 are F.
Preferred mixtures comprise not only one or more compounds of formulae IA and IB, but also one, two, three or more compounds of formulae IIa, IIb, IIc, IIf, IIIb, IIId, IIIf, IIIh, IIIi, IIIm, IIIs, IIIt or IIIu, preferably one or more compounds of formula IIIb, IIId, IIIh, IIIt or IIIu and from one to four compounds of formulae IA and IB and from one to three compounds of formulae IIa, IIb and/or IIc.
In the preferred compounds mentioned hereinabove and hereinafter of the subformulae relating to formulae II and III, R preferably is, unless noted otherwise, straight-chain alkyl, alkenyl or alkoxy, in particular alkyl, having from 1 to 12 C atoms, in particular having from 1 to 7 C atoms.
Also preferred are mixtures comprising one or more compounds of subformula IIIb1 
where R3 has the meaning specified in formula Ib.
R3 in the compounds of formula IIIb1 is particularly preferably vinyl, 1E-propenyl, 1E-butenyl, 3E-butenyl, 3E-pentenyl, particularly vinyl.
The individual compounds, e.g. of formulae II and III and their subformulae, or alternatively other compounds which can be used in the STN displays according to the invention, are either known or they can be prepared in a manner similar to that for the known compounds.
Preferred liquid crystal mixtures comprise one or more compounds of component A, preferably in a proportion of from 15% to 75%, particularly preferably from 20% to 65%. These compounds have a dielectric anisotropy xcex94∈xe2x89xa7+3, in particular xcex94∈xe2x89xa7+8, particularly preferably xcex94∈xe2x89xa7+12.
Preferred liquid crystal mixtures comprise one or more compounds of component B, preferably from 10 to 65%. The compounds of group B are distinguished, in particular, by their low values of rotational viscosity xcex31.
Component B preferably comprises one or more compounds selected from the group consisting of the compounds of formulae IV1 to IV9: 
where R1 and R2 have the meaning specified in formula IB.
Component B additionally comprises one or more compounds selected from the group consisting of the compounds of formulae IV10 to IV24, 
where R1 and R2 have the above-specified meaning and the 1,4-phenylene groups in IV10 to IV19, IV23 and IV24 can each, independently of one another, also be mono- or polysubstituted by fluorine.
Component B preferably comprises one or more compounds selected from the group consisting of the compounds of formulae IV25 to IV31: 
where R1 and R2 have the above-specified meaning and L is F or H. The 1,4-phenylene groups in the compounds IV25 to IV31 can each, independently of one another, also be mono- or polysubstituted by fluorine.
Particularly preferred are compounds of formulae IV25 to IV31, where R1 is alkyl and R2 is alkyl or alkoxy, particularly alkoxy, each having from 1 to 7 C atoms. Also preferred are compounds of formulae IV25 and IV31, where L is F.
R1 and R2 in the compounds of formulae IV1 to IV30 are particularly preferably straight-chain alkyl or alkoxy having from 1 to 12 C atoms.
Component B optionally comprises one or more compounds selected from the group consisting of the compounds of formulae VI and VII: 
where CrH2r+1 is a straight-chain alkyl group and r is an integer from 1 to 9.
In a particularly preferred embodiment, the mixtures comprise one or more alkenyl compounds of formula V, 
where
R3 is an alkenyl group having from 2 to 7 C atoms,
R4 has one of the meanings specified for R1 or, if m is 1, may alternatively be Qxe2x80x94Y,
Q is xe2x80x94CF2xe2x80x94, xe2x80x94OCF2xe2x80x94, xe2x80x94CFHxe2x80x94, xe2x80x94OCFHxe2x80x94 or a single bond,
Y is F or Cl,
m is 0 or 1, and
L1 and L2 each, independently of one another, are H or F.
Formula V encompasses the following compounds 
where R3, L1, L2, Q and Y have the above-specified meanings, and R4 has one of the meanings specified for R1.
Particularly preferred are TN and STN displays according to the invention, which comprise at least one compound of formula V-1 and/or V-3, particularly preferably at least one compound of each of formulae V-1 and V-3.
In the formulae V-1, V-2 and V-3, R3 particularly preferably is 1E-alkenyl or 3E-alkenyl having from 2 to 7 C atoms.
Preferred compounds of formula V-1 are those where R4 is alkenyl having from 2 to 7 C atoms. Particularly preferred are compounds of the following formulae 
where R3a and R4a each, independently of one another, are H, CH3, C2H5 or n-C3H7, and alkyl is an alkyl group having from 1 to 7 C atoms.
Particularly preferred are TN and STN displays according to the invention, in which the liquid crystal mixture comprises at least one compound of formulae V-1a and/or V-1c, in which R3a and R4a each have the same meaning, and displays in which the liquid crystal mixture comprises at least one compound of formula V-1e.
In a further preferred embodiment, the TN and STN displays comprise one or more compounds of formula V-2.
Particularly preferred compounds of formula V-2 are those where L1 and L2 are H, and those where R4 is alkyl having from 1 to 8, in particular 1, 2 or 3 C atoms, and R3 is 1E-alkenyl or 3E-alkenyl having from 2 to 7, particularly 2, 3 or 4 C atoms.
Most particularly preferred compounds of formula V2 are those of the following formulae 
where R3a, R4a and alkyl have the above-specified meaning, especially those where R3a is H.
Particularly preferred compounds of formula V-3 are those where L1 and/or L2 are F, and Qxe2x80x94Y is F or OCF3. Also preferred are compounds of formula I-3, where R3 is 1E-alkenyl or 3E-alkenyl having from 2 to 7, particularly 2, 3 or 4 C atoms.
The compounds of formulae V-1 and V-2 having a dielectric anisotropy of from xe2x88x921.5 to +1.5 are to be classified with the above-defined component B. The polar compounds of formulae V-2 and V-3 having a dielectric anisotropy of more than +1.5 are to be classified with the above-defined component A.
The use of compounds of formula V in the liquid crystal mixtures according to the invention results in particularly low values of rotational viscosity and in TN and STN displays having high steepness and rapid switching times, particularly at low temperatures.
In a further preferred embodiment, the liquid crystal mixtures according to the invention comprise not only the components A, B, C and D, but also one or more compounds from the group of the compounds of formulae VIII and IX 
where R1 and R2 have the above-specified meanings.
Further preferred are liquid crystal mixtures comprising at least one component selected from the group consisting of the compounds of formulae X to XIV: 
where Hal is F or Cl, and L is H or F, and R has the above-specified meaning, particularly where R is alkyl having from 1 to 12 C atoms.
The liquid-crystalline mixtures optionally comprise an optically active component D in such an amount that the ratio between layer thickness (spacing of the substrates) and natural pitch of the chiral nematic liquid crystal mixture is greater than 0.2. For this component, those skilled in the art have at their disposal a multiplicity of chiral dopants, some of which are commercially available, e.g. cholesteryl nonanoate, S-811 from Merck KGaA, Darmstadt, and CB15 (BDH, Poole, UK). The choice of dopants is not critical per se.
The proportion of compounds of component D is preferably 0 to 10%, in particular from 0 to 5%, particularly preferably from 0 to 3%.
In a particularly preferred embodiment, the mixtures according to the invention, in addition to the compounds of formula IB, comprise from about 2 to 65%, particularly from 5 to 35%, of liquid-crystalline tolane compounds. This allows smaller layer thicknesses to be employed, resulting in distinctly shorter switching times. The tolane compounds are preferably selected from group T consisting of the compounds of formulae T1 and T2: 
where 
xe2x80x83is 
xe2x80x83or 
xe2x80x83preferably 
xe2x80x83in formula T1 alternatively 
xe2x80x83in formula T2 alternatively 
d is 0 or 1,
L1 to L7 each, independently of one another, are H or F,
Q is xe2x80x94CF2xe2x80x94, xe2x80x94CHFxe2x80x94, xe2x80x94OCF2xe2x80x94, xe2x80x94OCHFxe2x80x94 or a single bond,
Y is F or Cl,
Z4 is xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94CH2CH2xe2x80x94 or a single bond, and
R1 and R2 have the above-specified meanings.
Preferred compounds of formula T1 correspond to the subformulae T1a and T1b 
where L1 to L4 are H or F, and Qxe2x80x94Y is F, Cl or OCF3, particularly F or OCF3.
Preferred compounds of formula T2 correspond to the subformulae T2a to T2g 
where R1, R2 and Z4 have the above-specified meanings, and L1 to L6 are H or F.
Particularly preferred compounds of formula T2e are those where one, two or three of the radicals L1 to L6 are F and the others are H, L1 and L2 or L3 and L4 or L5 and L6 not simultaneously being F.
The proportion of the compounds from group T is preferably from 2 to 65%, particularly from 5 to 35%.
The mixtures according to the invention may optionally comprise up to 20% of one or more compounds having a dielectric anisotropy of  less than xe2x88x922 (component C).
If the mixtures comprise compounds of component C, these are preferably one or more compounds having the structural element 2,3-difluoro-1,4-phenylene, e.g. compounds according to DE-A-38 07 801, 38 07 861, 38 07 863, 38 07 864 or 38 07 908. Particularly preferred are tolanes having said structural element according to international patent application PCT/DE 88/00133, particularly the abovementioned tolanes of formula T2 and their preferred subformulae.
Further known compounds of component C are, for example, derivatives of the 2,3-dicyanohydroquinones or cyclohexane derivatives having the structural element 
or 
according to DE-A-32 31 707 and DE-A-34 07 013.
Preferably, the liquid crystal displays according to the invention do not comprise any compounds of component D.
The liquid crystal mixture according to the invention preferably comprises one or more compounds selected from group B1 consisting of compounds of formulae B1I to B1IV: 
where
R1, R2 and Z4 have the above-specified meanings, and 
xe2x80x83is 
xe2x80x83or 
and/or at least one compound selected from group B2 consisting of compounds of formulae B2I to B2III: 
where
R has the above-specified meaning,
Z5 is xe2x80x94CH2CH2xe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94 or a single bond,
Q2 is 
xe2x80x83or 
xe2x80x83alkyl is an alkyl group having from 1 to 9 C atoms,
X is CN or F, and
L is H or F,
and/or at least one compound selected from group B3 consisting of compounds of formulae B3I to B3III: 
where
R1 and R2 independently of one another have the above-specified meanings,
Y is F or Cl, and 
xe2x80x83is 
xe2x80x83or 
The proportion of the compounds from group B1 is preferably from 10 to 50%, particularly from 15 to 40%. Compounds of formulae B1III and B1IV are preferred.
Particularly preferred compounds of group B1 are those having the following component formulae, 
where
R1a is CH3xe2x80x94(CH2)pxe2x80x94, CH3xe2x80x94(CH2)pxe2x80x94Oxe2x80x94, CH3xe2x80x94(CH2)pxe2x80x94Oxe2x80x94CH2xe2x80x94, trans-Hxe2x80x94(CH2)qxe2x80x94CHxe2x95x90CHxe2x80x94(CH2CH2)sxe2x80x94CH2Oxe2x80x94 or trans-Hxe2x80x94(CH2)qxe2x80x94CHxe2x95x90CHxe2x80x94(CH2CH2)sxe2x80x94,
R2a is CH3xe2x80x94(CH2)pxe2x80x94,
p is 1, 2, 3 or 4,
q is 0, 1, 2 or 3, and
s is 0 or 1.
The proportion of compounds of the above-specified component formulae B1IIIa and B1IIIb together with the compounds of formula IB1 is preferably from about 5 to 45%, particularly preferably from about 10% to 35%.
The proportion of compounds of component formula B1IVa or compounds of formula B1IV is preferably from about 5 to 40%, particularly preferably from about 10 to 35%.
In a particularly preferred embodiment, the mixtures simultaneously comprise compounds of formulae B1III and B1IV together with the compounds of formulae IB1 and IB2, the overall proportion for components of group B1 being maintained.
If compounds of formulae B1I and/or B1III are present, R1 and R2 each preferably are, independently of one another, n-alkyl having from 1 to 7 C atoms or (trans)-n-alkenyl having from 3 to 7 C atoms. Z is preferably a single bond.
Also preferred are mixtures according to the invention which comprise one or more compounds of formula B1IV, where 
is 
or 
and R1 and R2 have one of the above-specified preferred meanings and, in particular, preferably n-alkyl having from 1 to 7 C atoms.
In each instance, the overall proportion for components of group B1 is maintained.
The proportion of compounds of group B2 is preferably from about 5 to 45%, in particular from 5 to 20%. The proportion (preferred ranges) for B2I to B2III is as follows:
B2I: from about 5 to 30%, preferably from about 5 to 15%,
Sum of B2II and B2III: from about 5 to 25%, preferably from about 10 to 20%.
Preferred compounds of group B2 are listed below: 
where R1, R2, L and Z5 have the above-specified meanings.
R1 in these compounds is preferably n-alkyl having from 1 to 7 C atoms or (trans)-n-alkenyl having from 3 to 7 C atoms, Z5 is preferably a single bond, R2 preferably has the preferred meaning specified hereinabove for R or is fluorine, L is preferably fluorine.
Preferably, the mixtures according to the invention comprise one or more compounds selected from the group consisting of B2Ic, B2IIa and B2IIIa in an overall proportion of from about 5 to 35%.
In a particularly preferred embodiment, the mixtures above the invention comprise not only B2Ic, B2IIa, B2IIIa (Lxe2x95x90F), but also further terminally fluorinated compounds, for example selected from the group consisting of 
and/or polar heterocyclic compounds selected from the group consisting of 
where R1 is preferably n-alkyl having from 1 to 7 C atoms or (trans)-n-alkenyl having from 3 to 7 C atoms, g is 1 or 2, h is 0 or 1, X0 is F, Cl, CF3, xe2x80x94OCF3 or xe2x80x94OCHF2, and L is H or F.
The overall proportion of all the terminally fluorinated compounds is preferably from about 5 to 65%, in particular from about 15 to 40%.
The proportion of compounds from group B3 is preferably from about 5 to 30%, particularly preferably from about 10 to 20%. R1 is preferably n-alkyl or n-alkoxy, either having from 1 to 9 C atoms.
Alternatively it is possible, however, for analogous compounds containing alkenyl or alkenyloxy groups to be used. Compounds of formula B3I are preferred.
The term xe2x80x9calkenylxe2x80x9d in the meaning of R, R1, R2, R3 and R4 encompasses straight-chain and branched alkenyl groups, having 2-12 carbon atoms in the case of R, R1 and R2, and having 2-7 carbon atoms in the case of R3 and R4, particularly the straight-chain groups. Particularly preferred alkenyl groups are C2-C7-1E-alkenyl, C4-C7-3E-alkenyl, C5-C7-4-alkenyl, C6-C7-5-alkenyl, and C7-6-alkenyl, particularly C2-C7-1E-alkenyl, C4-C7-3E-alkenyl and C5-C7-4-alkenyl.
Examples of preferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5 carbon atoms are generally preferred.
In a preferred embodiment, the liquid-crystalline media according to the invention comprise 3, 4, 5 or 6 compounds of formulae IA and IB; the proportion of these compounds as a rule is from 10 to 65 wt %, preferably from 15 to 50 wt %, based on the mixture as a whole.
In further preferred embodiments, the mixtures comprise
one or more compounds of formula IB where L is F,
one or more compounds of formula IA-1 
xe2x80x83especially those where R3a is H or CH3, most particularly preferably CH3,
one or more compounds of formula IA-2 
xe2x80x83especially those where R3a is H or CH3, most particularly preferably H,
one or more compounds of the following formulae 
where R1, R2 and L have the preferred meanings mentioned under compounds of component B. The proportion of these compounds in the liquid crystal mixtures is preferably from 0 to 45%, in particular from 5 to 30%,
one or more, particularly 1, 2, 3 or 4 compounds, selected from the compounds of formulae IIIb, IIId, IIIf, IIIh, IIIi, IIIm, IIIs, IIIt and IIIu;
at least two compounds selected from the compounds of formulae IIb1, IIb2, IIc1 and IIc2. The proportion of these compounds in the liquid crystal mixtures is preferably from 0 to 60%, particularly from 10 to 45%;
at least one compound of formulae V-1, V-2 and/or V-3, particularly preferably at least one of each, in particular from 2 to 6, compounds selected from the following formulae: 
where R3a and R4a each, independently of one another, are H, CH3, C2H5 or n-C3H7, R3a in particular is H, and alkyl is an alkyl group having from 1 to 7 C atoms,
from 8 to 30%, particularly from 10 to 25%, of one or more compounds of formula IA,
from 6 to 35%, particularly from 8 to 30%, of one or more compounds of formula IB.
Further particularly preferred embodiments relate to liquid crystal mixtures which
comprise a total of from 2 to 6 compounds of formulae IA and IB, the proportion of these compounds in the overall mixture being from 15 to 65%, in particular from 20 to 55%,
comprise at least 20%, in particular from 25 to 90%, particularly preferably from 32 to 80% of alkenyl compounds, particularly those of formulae IB and V,
comprise more than 20% of compounds having a dielectric anisotropy xcex94∈xe2x89xa7+12,
essentially consist of compounds of formulae IA, IB, V, IIc and/or IId and optionally additionally IV25 and IV30.
The mixtures according to the invention are distinguished, particularly when used in TN and STN displays having high layer thicknesses, by very low total switching times (ttot=ton+toff). Low total switching times are an important criterion particularly for STN displays when used as laptop displays, to allow cursor movements to be shown in a trouble-free manner.
The liquid crystal mixtures used in the STN and TN cells according to the invention are dielectrically positive with xcex94∈xe2x89xa71. Particularly preferred are liquid crystal mixtures having xcex94∈xe2x89xa73, particularly those having xcex94∈xe2x89xa75.
The liquid crystal mixtures according to the invention exhibit useful values for the threshold voltage V10/0/20 and for the rotational viscosity xcex31. If the optical path difference dxc2x7xcex94n has a predetermined value, the value of the layer thickness d is defined by the optical anisotropy xcex94n. Particularly for relatively high values of dxc2x7xcex94n, the use of liquid crystal mixtures according to the invention having a relatively high value of the optical anisotropy is preferred, since a relatively small value of d can then be chosen, leading to more favourable values of the switching times. However, even those liquid crystal displays according to the invention which comprise liquid crystal mixtures according to the invention having smaller values of xcex94n are characterized by advantageous values of the switching times.
The liquid crystal mixtures according to the invention are further characterized by advantageous values of the slope of the electro-optical characteristic curve and can be operated at high multiplex rates, especially at temperatures above 20xc2x0 C. Moreover, the liquid crystal mixtures according to the invention exhibit high stability and favourable values of the electrical resistance and the threshold voltage frequency dependence. The liquid crystal displays according to the invention have a wide operating temperature range and good angular dependence of contrast.
The configuration of the liquid crystal display elements according to the invention, comprising polarizers, electrode baseplates and electrodes surface-treated in such a way that the preferential alignment (director) of the liquid crystal molecules adjoining each of these is usually twisted with respect to one another by an amount of from 160xc2x0 to 720xc2x0, corresponds to the design customary for such display elements. In this context, the term xe2x80x9ccustomary designxe2x80x9d is to be interpreted broadly and also encompasses all alterations and modifications of the TN and STN cell, including matrix display elements in particular, and the display elements comprising additional magnets.
The surface tilt angle at the two substrates can be identical or different. Identical tilt angles are preferred. Preferred TN displays have pre-tilt angles between the longitudinal axis of the molecules at the surface of the substrates and the substrates of from 0xc2x0 to 7xc2x0, preferably from 0.01xc2x0 to 5xc2x0, in particular from 0.1 to 2xc2x0. In the STN displays the pre-tilt angle is from 1xc2x0 to 30xc2x0, preferably from 1xc2x0 to 12xc2x0 and in particular from 3xc2x0 to 10xc2x0.
The twist angle of the TN mixture in the cell amounts to between 22.5xc2x0 and 170xc2x0, preferably between 45xc2x0 and 130xc2x0 and in particular to between 80xc2x0 and 115xc2x0. In the display, the twist angle of the STN mixture, from alignment layer to alignment layer, amounts to between 100xc2x0 and 600xc2x0, preferably to between 170xc2x0 and 300xc2x0 and in particular to between 180xc2x0 and 270xc2x0.
The threshold voltage of the liquid crystal displays containing the nematic liquid crystal mixtures according to the instant application are preferably less than or equal to 1.85 V, more preferably less than or equal to 1.70 V and most preferably less than or equal to 1.65 V and in particular less than 1.40 V. These conditions apply for liquid crystal cells with a twist angle of 240xc2x0 and a tilt angle of 4xc2x0 or more.
Under these conditions the steepness (V90/V10) is preferably less than or equal 1.08, more preferably less than or equal to 1.07, more preferably less than or equal to 1.06 and most preferably less than or equal to 1.05.
The preparation of the liquid crystal mixtures usable according to the invention is performed in a manner customary per se. As a rule, the desired quantity of the components used in a smaller amount is dissolved in the components constituting the main constituent, expediently at elevated temperature. Alternatively it is possible to mix solutions of the components in an organic solvent, e.g. in acetone, chloroform or methanol and to remove the solvent again after thorough mixing, for example by distillation.
The dielectrics may further comprise additives known to those skilled in the art and described in the literature. For example, 0-15% of pleochroitic dyes can be added.
The entire disclosure of all applications, patents and publications, cited above or below, and of corresponding German Application No. 199 30 211.1, filed Jun. 30, 1999 is hereby incorporated by reference.